This invention relates generally to gas anesthesia machines, and, more particularly, to subsystems in such machines for adding vapors to a mixture of anesthesia gas and oxygen.
A gas anesthesia machine includes an oxygen supply, a supply of anesthesia gas, such as a nitrous oxide (N.sub.2 O), flow control valves and flow meters for the anesthesia gas and the oxygen, and a common outlet by means of which a mixture of the gases is passed to a patient breathing machine. The patient breathing machine, which forms no part of the present invention, is typically a closed-circuit system including a carbon dioxide absorber and at least two check valves, to ensure that the patient inhales gas from the common outlet and exhales through the carbon dioxide absorber. The patient breathing machine may also include a ventilator to pump breathing gas into the patient's lungs, and a gas evacuation system for removal of excess gas. In simple terms, a gas anesthesia machine provides a mixture of anesthesia gas and oxygen in proportions selected by an operator, who is usually a doctor.
On many occasions, the doctor or anesthesiologist wishes to add another substance in vapor form to the mixture of anesthesia gas and oxygen. For this purpose, the gases are passed through a vaporizer from which the added substance is removed, usually by evaporation, before passing to the common outlet and thence to the patient breathing machine.
There are a number of different specific types of vaporizers, but only two general types with which this invention is concerned. First there are direct reading vaporizers. As the name implies, a direct reading vaporizer has a control knob graduated in terms of the percentage concentration of the vapor that is added to a gas stream passing through the vaporizer. A direct reading vaporizer is inserted into the oxygen and anesthesia gas flow immediately prior to the common outlet, to allow the addition of a selected concentration of vapor to the combined gases. The other type of vaporizer with which the invention is concerned is the universal vaporizer, sometimes referred to as a copper kettle vaporizer. The universal vaporizer adds vapor to an independently controlled oxygen circuit, i.e., a separate circuit comprising a flow control valve and flow meter. The universal vaporizer flow can be calibrated in advance and can be used independently of the main oxygen and anesthesia gas flow. Depending on the requirements for a particular surgical procedure, the doctor may have need for one or more direct reading vaporizers at various times, as well as for a universal vaporizer. Accordingly, there is a need to provide for convenient switching from one vaporizer to another, while ensuring at all times that only one vaporizer is connected to the patient breathing machine any particular time.
Some systems utilize the off position of direct reading vaporizers to ensure that only one vaporizer is not connected at any particular time. Although this should theoretically isolates unused vaporizers from the rest of the system, there is always the possibility of a malfunction of the vaporizer controls, so that a vaporizer in the off position could still add vapor to the diluent stream of gases from the gas anesthesia machine. Ideally, then, the unused vaporizers should be completely isolated from the diluent flow, even when a faulty control leaks in the off position. One prior art technique of providing such isolation is to mount the vaporizers in a rotatable turret so constructed that only one vaporizer at a time can be connected into the diluent flow. Such a system utilizes quick disconnect devices to disconnect one vaporizer and reconnect another after rotating the turret to the appropriate position. Although such a system provides isolation and inherently prevents the use of more than one vaporizer, the system is relatively inconvenient to use and still relies on the off position of the control knob for the vaporizer that is connected to the system. Thus, the isolation provided by a turret system is not reliably complete, since a malfunction of the connected vaporizer could still add vapor to the diluent flow.
It will be apparent from the foregoing discussion that there are basically three requirements for an ideal vaporizer subsystem. First, there should be isolation of all unused vaporizers, even when all vaporizers are unused. Second, there should be a convenient and reliable interlock device to ensure that only one vaporizer may be selected at any time, whether the selected vaporizer is a direct reading vaporizer or a vaporizer of the universal type. Finally, there should be a clear visual indication, preferably visible from some distance, of which vaporizer is connected to the diluent flow. The present invention is directed to a vaporizer subsystem that satisfies these three ideal requirements and overcomes the disadvantages of prior subsystems of this type.